1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine including a first fuel injection mechanism (in-cylinder injector) injecting fuel into a cylinder and a second fuel injection mechanism (intake manifold injector) injecting fuel towards an intake manifold or an intake port, and particularly to a control apparatus for feedback control of the air-fuel ratio of the exhaust system prior to the catalyst to the stoichiometric air-fuel ratio.
2. Description of the Background Art
An internal combustion engine is well-known, including an intake manifold injector for injecting fuel into the intake manifold of the engine and an in-cylinder injector for injecting fuel into the engine combustion chamber, wherein fuel injection from the intake manifold injector is inhibited when the engine load is lower than a predetermined set load, and fuel injection from the intake manifold injector is conducted when the engine load is higher than the set load.
In general, a catalytic converter for purifying noxious components in the exhaust gas is provided at the exhaust system of an internal combustion engine. A three-way catalytic converter is generally used as such a catalytic converter. The three-way catalytic converter oxidizes carbon monoxide (CO) and unburned hydrocarbon (HC) and reduces nitride oxide (NOx) that are the three noxious components in the exhaust gas to turn them into innoxious carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2).
The purifying performance of the three-way catalytic converter depends upon the air-fuel ratio of the air-fuel mixture developed in the combustion chamber. The three-way catalytic converter functions most effectively when the air fuel ratio is in the vicinity of the stoichiometric air-fuel ratio. This is because all these three noxious components set forth above cannot be purified favorably since oxidation is active whereas reduction is inactive when the air-fuel ratio is lean and the amount of oxygen in the exhaust gas is high, and the reduction is active whereas oxidization is inactive when the air-fuel ratio is rich and the amount of oxygen in the exhaust gas is low. Therefore, an internal combustion engine including a three-way catalytic converter has an output linear type oxygen sensor provided at the exhaust manifold. Feedback control is executed such that the air-fuel ratio of the air-fuel mixture in the combustion engine corresponds to the stoichiometric air-fuel ratio (the ideal air-fuel mixture ratio; hereinafter also termed stoichiometric ratio) based on the oxygen concentration measured by the oxygen sensor.
Japanese Patent Laying-Open No. 11-351011 discloses a fuel injection control apparatus for an internal combustion engine that includes an auxiliary fuel injection valve that allows fuel to be injected into the intake manifold in addition to a main fuel injection valve for directly injecting fuel into the combustion chamber. The auxiliary fuel injection valve is operated under a predetermined operation condition. In the case where the main fuel injection valve and the auxiliary fuel injection valve partake in fuel injection into the internal combustion engine, control is effected to prevent erroneous learning by a temporary error in air-fuel ratio at the time of switching between operation and non-operation of the auxiliary fuel injection valve. This control apparatus corresponds to a fuel injection control apparatus for a direct-injection spark plug type internal combustion engine including a main fuel injection valve to inject fuel directly into the combustion chamber. This control apparatus includes basic fuel injection quantity calculation means for calculating the basic fuel injection quantity based on the operation condition of the engine, air-fuel ratio feedback correction coefficient setting means for setting by increasing/decreasing the air-fuel ratio feedback correction coefficient according to whether the air-fuel ratio detected by the air-fuel ratio sensor is rich or lean at a predetermined air-fuel ratio feedback control condition, a rewritable learning correction coefficient storage means for storing a learning correction coefficient, fuel injection quantity calculation means for calculating the fuel injection quantity based on the basic fuel injection quantity, the air-fuel ratio feedback correction coefficient, and the learning correction coefficient, and learning means for updating the learning correction coefficient in a direction approximating the reference value based on the air-fuel ratio feedback correction coefficient by learning at a predetermined learning condition. This control apparatus further includes switching control means for operating the auxiliary fuel injection valve at a predetermined operation condition such that the main fuel injection valve and the auxiliary fuel injection valve partake in fuel injection into the internal combustion engine. Learning inhibition means is provided to inhibit learning based on the learning means for a predetermined period at the time of switching between an operating and non-operating state of the auxiliary fuel injection valve.
The fuel injection control apparatus for an internal combustion engine set forth above is advantageous in that the learning accuracy is improved since erroneous learning of a temporary error in air-fuel ratio at the time of switching between an operating and non-operating state of the auxiliary fuel injection valve is eliminated.
Although the convergence towards a target value is accelerated as the gain of feedback control becomes higher, there is the possibility of oscillation when the gain is too high. This gain depends upon the inefficient time and/or delay in response of the control system. The gain can be set higher as the inefficient time and/or delay in response becomes smaller to allow increase in response to the target value.
In the apparatus disclosed in the aforementioned Japanese Patent Laying-Open No. 11-351011, the required injection quantity is calculated based on the basic fuel injection quantity and the correction by feedback control. The required injection quantity is multiplied by the fuel injection ratio of the fuel injection valve (in-cylinder injector) to the auxiliary fuel injection valve (intake manifold injector) to calculate the fuel injection quantity of the in-cylinder injector and the fuel injection quantity of the intake manifold injector. The fuel injected from the intake manifold injector will adhere to the inner wall of the intake manifold, causing delay in response. Since a high gain cannot be set, the gain used in calculating the value of correction by feedback control had to be set at a low level. It was therefore difficult to increase the response to the target value.
The tendency of causing delay in response due to the wall adherence of fuel injected from the intake manifold injector becomes more significant as the intake manifold is under a cold state. Therefore, the gain must be modified depending upon the temperature. This means that, even if the amount of fuel injected from the intake manifold injector is increased to achieve a rich state from the lean state, a high gain cannot be set since the delay is serious due to the effect of adherence at the wall when the temperature is low. Thus, favorable response could not be realized.